The active noise cancellation, also called active noise compensation, is used, for example, in headphones, earphones, or telephones, in order to damp undesired and disruptive background noise and to reproduce the wanted sound so that it is more easily understandable. Therefore, for a user, a clear quality improvement of the reproduced speech and/or music is achieved. In order to achieve active noise cancellation, there are two different approaches, namely feedback and feed forward. The approach forming the basis of this application is based on feedback.
In a known system with active noise compensation by feedback, a microphone is positioned in front of a loudspeaker. The microphone measures the ambient noise. The microphone signal is converted in a feedback loop (i.e., a control loop with feedback) and fed together with a wanted signal to the loudspeaker. The signal feedback and preparation causes suppression of the ambient noise.
In the case of feedback, with respect to the frequency, phase shifts occur, wherein a so-called phase margin that is required for a stable operation of the feedback loop becomes limited. These frequency-dependent phase shifts are caused, for example, by the loudspeaker, the microphone, the acoustic properties of the housing surrounding the microphone and loudspeaker, and the distances between the individual components.
Measurements have shown that, in particular, the distance between the loudspeaker and microphone causes a time delay that results in a linear phase shift as a function of frequency. This time delay, which significantly reduces the phase margin at high frequencies, cannot be compensated in feedback. In the case of a phase shift of greater than 180°, the negative feedback transforms into feed forward and the feedback loop begins to oscillate. From the measurements, it is clear that a larger distance between the loudspeaker and microphone causes a larger phase shift.